Because of the compact nature of outboard motors, the design of many of the components of the outboard motor is made quite difficult. A specific area where the size constraints present problems is in the design of the exhaust system for the outboard motor. In many vehicle applications, the exhaust system which transfers the exhaust gases from the engine to the atmosphere can have its length and shape varied within large parameters due to the nature of the vehicle that is powered. However, with outboard motors the exhaust system must be very compact and hence the silencing efficiency and tuning becomes quite a problem. For this reason, it is the normal practice to discharge the exhaust gases to the atmosphere, through an underwater exhaust gas discharge, under most running conditions. In this way, the exhaust gases are cooled and silenced by the body of water in which the watercraft is operating.
Also, it has been proposed to introduce the cooling water from the engine cooling jacket into the exhaust system so as to assist in the silencing and cooling of the exhaust gases. Cooling of the exhaust gases is important because this not only aids in the silencing, but it protects surrounding components from the heat of the exhaust. This heat transfer is a particular problem because of the compact nature of the outboard motor.
However, with increasing emphasis on protection of the environment, it is necessary or desirable to employ catalytic treatment systems for the exhaust gases. The catalyst can be utilized to remove harmful constituents from the exhaust gases or to transform them into less harmful products before they are discharged. Since the exhaust gases are also delivered to the atmosphere through the body of water in which the watercraft is operating, the control of water pollution also may dictate the desirability of employing catalysts.
However and is well known, the catalyst normally operates at a relatively high temperature to be effective. Also, the material of the catalyst is such that if it is cooled suddenly, the catalyst bed may fracture. Also, any water which may come in contact with the catalyst can pollute the catalyst by leaving deposits when the water evaporates. These problems are particularly acute in conjunction with operation in marine environments. The salt water can leave salt deposits on the catalyst that can foul it and reduce its capability of performing its intended functions.
In order to protect the catalyst from water damage, particularly when considering the fact that the exhaust gases have a discharge outlet that is below the water level, it is desirable to position the catalyst relatively high in the outboard motor. Such high locations will assist in protecting the catalyst. However, if the catalyst is positioned in close proximity to the power head, then the heat transfer problems become particularly acute. This is particularly true since a portion of the protective cowling of the power head is formed from a nonmetallic material, such as a molded fiberglass reinforced resin or the like. These materials are quite susceptible to damage under high temperatures.
It is also desirable to mount the catalyst in close proximity to the exhaust ports of the engine. This is because the catalyst should be at a high temperature in order to perform its operation. Thus, with the foregoing problems in mind, it has been proposed to mount a catalyst on the underside of an exhaust guide plate that receives the exhaust gases directly from the exhaust manifold of the engine. However, this positioning of the catalyst within the drive shaft housing raises a possibility of its being contacted by water with the aforenoted deleterious affects. Therefore, it has been proposed to employ certain protective devices, including the enclosing of the catalyst within a closed chamber in the drive shaft housing.
With all of these protection methods, there still is a possibility that the catalyst may become contaminated in use. This is particularly true when operating in marine environments because it is difficult, if not impossible, to totally isolate the catalyst from water vapor in the atmosphere.
If the catalyst becomes fowled with deposits, such as salt, it is rendered less effective, as aforenoted. A primary method of determining if the catalyst is operating effectively is to analyze the exhaust gases.
It is, therefore, an object of the present invention to provide an improved catalytic exhaust system for an outboard motor which conveniently permits the testing of the exhaust gases to determine if they are being properly catalyzed. In addition, it is an object of the invention to provide such a system which further including a means for sensing the exhaust gas temperature.